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Monday, 6 December 2010

Reconstructing the ice edge in Somerset

What did the ice edge look like when the Irish Sea Glacier came inland from the Bristol Channel?  There are a number of options -- above are two examples from Greenland. 

Let's imagine that this is the Irish Sea Glacier, pushing inland into Somerset from the NW.  The one photo (from Google Earth) shows incredibly clean ice with virtually no morainic material and a scraped and eroded bedrock floor.  This is from North Greenland -- here there is deep and continuous permafrost and virtually no bottom melting beneath the ice sheet.  The glacier is probably frozen to its bed, and moves largely through internal deformation.  There is probably not much erosion going on today.

The other photo is from South Greenland, showing ice that is rapidly on the way out.  It is dirty and thin, and there is water everywhere.  Here the glacier is clearly sliding and is wet-based, so there is probably much erosion up-glacier and a vast amount of deposition.

Of course, during a glaciation conditions can swing from one extreme to another, with cold and clean ice at the peak (ie coldest part) of the glacial episode, and melting, erosion and dumping of debris towards the end of the glaciation, as the climate warms and the ice edge goes into retreat.

Wonderful pictures, don't you think?  Click to see them in more detail.


Kosta Dean said...

Brian, these photos clearly show that “not all ice is created equal”. This is a point of argument I've been making for some time now, in your blog and in my paper “The un-Henging of Stonehenge”.

I think that too much has been made of “glacier ice” and the evidence or lack of evidence of such at Salisbury Plain. But if we consider that ice can form 'locally' and not flow in an area through glaciation (as for example by solidly frozen lakes or rivers) we can explain all the morphology associated with Stonehenge and other prehistoric sites.

The periglacial stripes excavated at the Avenue clearly open the possibility that this area was experiencing a deep freeze. We know scientifically that there was a sharp drop in temperature around 10,000 BC with the Little Ice Age that griped northern Europe. We also know that Avon River was some 30 m higher or more than it is today. How unreasonable would it be to argue that all of Salisbury Plain was submerged by water (what Robert argues) and that this body of water was frozen (what I argue)?

This is far better than attributing supernatural capabilities to Neolithic men and inventing navigational skills and sail boats that did not come to historical existence till 5,000 years later (sorry Robert!) Any argument that depends on a disconnect with history or science is no different than the argument that extra terrestrials built Stonehenge (as well as the Pyramids); or the Wizard Merlin built Stonehenge. Such theories rest on unswerving belief and are not 'falsifiable', as is required by science and common sense.

Constantinos Ragazas

BRIAN JOHN said...

I'm perfectly happy to try and explain that glacier ice comes in a number of different "formats" -- as people have recognized for well over a century. Polar ice. temperate ice, wet-based ice, frozen-on ice, etc etc -- I am trying to work out what the characteristics of the ice were when the Irish Sea Glacier pressed in from the west. But whatever the internal thermal characteristics of the ice may be, it always follows the same physical laws.

When frozen ground exists, and there is reasonable precipitation of snow, you will get snowpatches, frozen water bodies etc in an undulating area like Salisbury Plain. If it is very cold and arid, nothing much will happen, and the snow accumulation will not give you a glacier. (In a glacier, there is genuine glacier ice, and the ice body MOVES.) But if accumulation continues unabated, then a plateau snowfield can become a small ice-cap, and this will involve some movement of ice out from the centre. This is what I envisage for Dartmoor and Exmoor for the "waxing" phases of the big glaciations -- whether it happened on Salisbury Plain and on the Downs, Cotswolds, Mendips, Chilterns etc is a matter of debate. It is possible that there were a number of small ice caps which remained semi-independent, and which were too thin to move -- so they may have just sat on their hilly areas and effectively protected them. That much is reasonable......

I doubt that the Younger Dryas (Zone 3) cooling was prolonged enough to create such small ice-caps in Southern England. The Little Ice Age was different -- much later, causing frozen rivers and lakes and much hardship in the 1700s and 1800s.

Please forget about the Avon terraces. They have nothing to do with local shorelines and water bodies. The idea that Salisbury Plain was -- in part -- covered by water around 7,000 years ago is not supported by a single scrap of evidence, as far as I can see.